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.Dd December 1, 2017
.Dt DUP 2
.Os
.Sh NAME
.Nm dup ,
.Nm dup2
.Nd duplicate an existing file descriptor
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn dup "int oldd"
.Ft int
.Fn dup2 "int oldd" "int newd"
.Sh DESCRIPTION
The
.Fn dup
system call
duplicates an existing object descriptor and returns its value to
the calling process
.Fa ( newd
=
.Fn dup oldd ) .
The argument
.Fa oldd
is a small non-negative integer index in
the per-process descriptor table.
The new descriptor returned by the call
is the lowest numbered descriptor
currently not in use by the process.
.Pp
The object referenced by the descriptor does not distinguish
between
.Fa oldd
and
.Fa newd
in any way.
Thus if
.Fa newd
and
.Fa oldd
are duplicate references to an open
file,
.Xr read 2 ,
.Xr write 2
and
.Xr lseek 2
calls all move a single pointer into the file,
and append mode, non-blocking I/O and asynchronous I/O options
are shared between the references.
If a separate pointer into the file is desired, a different
object reference to the file must be obtained by issuing an
additional
.Xr open 2
system call.
The close-on-exec flag on the new file descriptor is unset.
.Pp
In
.Fn dup2 ,
the value of the new descriptor
.Fa newd
is specified.
If this descriptor is already in use and
.Fa oldd
\*(Ne
.Fa newd ,
the descriptor is first deallocated as if the
.Xr close 2
system call had been used.
If
.Fa oldd
is not a valid descriptor, then
.Fa newd
is not closed.
If
.Fa oldd
==
.Fa newd
and
.Fa oldd
is a valid descriptor, then
.Fn dup2
is successful, and does nothing.
.Sh RETURN VALUES
These calls return the new file descriptor if successful;
otherwise the value -1 is returned and
the external variable
.Va errno
is set to indicate the cause of the error.
.Sh ERRORS
The
.Fn dup
system call fails if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa oldd
argument
is not a valid active descriptor
.It Bq Er EMFILE
Too many descriptors are active.
.El
.Pp
The
.Fn dup2
system call fails if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa oldd
argument is not a valid active descriptor or the
.Fa newd
argument is negative or exceeds the maximum allowable descriptor number
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr close 2 ,
.Xr fcntl 2 ,
.Xr getdtablesize 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr socket 2 ,
.Xr socketpair 2 ,
.Xr dup3 3
.Sh STANDARDS
The
.Fn dup
and
.Fn dup2
system calls are expected to conform to
.St -p1003.1-90 .
.Sh HISTORY
The
.Fn dup
function appeared in
.At v3 .
The
.Fn dup2
function appeared in
.At v7 .
